Such a roller-type friction transmission unit is known as is described in a patent document 1. The roller-type friction transmission unit described in this document includes a pair of rollers, wherein an outer peripheral surface of a first roller and an outer peripheral surface of a second roller are pressed in a radial direction of the first and second rollers into pressing contact with one another, to allow frictional power transmission between the first and second rollers; the first roller is connected to a driveline to main driving wheels of a four wheel drive vehicle; and the second roller is connected to a driveline to auxiliary driving wheels of the four wheel drive vehicle.
In the case of this roller-type friction transmission unit, if the outer peripheral surfaces of the rollers in contact with one another are cylindrical surfaces, the outer peripheral surfaces are in line contact with one another. Accordingly, if the parallelism between the roller rotation axes is lowered by errors in assembling or errors in precision of parts, it significantly adversely affects the endurance and transmission efficiency of the roller-type friction transmission unit. In this way, it is disadvantageous in view of robustness.
Accordingly, in a typical roller-type friction transmission unit, the outer peripheral surface of one roller is formed to have a relatively raised portion middle in the axial direction, so that the outer peripheral surfaces of the rollers are in point contact with each other. In such a case, even if the parallelism between the roller rotation axes is lowered by errors in assembling or errors in precision of parts, the point contact between the outer peripheral surfaces of the rollers is maintained so that it does not significantly adversely affect the endurance and transmission efficiency of the roller-type friction transmission unit. In this way, it is superior in robustness.
However, in the case of the conventional roller-type friction transmission unit described above, the configuration that roller outer peripheral surfaces are in frictional contact with each other at a single point may raise the contact pressure between the rollers at that contact point, and thereby cause the following problem.
A roller-type friction transmission unit has a characteristic of change of traction coefficient (μ) with respect to inter-roller slip speed (V), which is so-called the μ-V characteristic, in which characteristic, traction coefficient (μ) rapidly rises to a maximum value immediately after the occurrence of inter-roller slip speed (V), and thereafter traction coefficient (μ) gradually decreases from the maximum value as inter-roller slip speed (V) increases.
Although that tendency of μ-V characteristic is unchanged, the ideal slip speed (Vo) that is inter-roller slip speed (V) when traction coefficient (μ) reaches the maximum value, the maximum value (μmax) of traction coefficient (μ), the range of negative gradient in which traction coefficient (μ) gradually decreases from the maximum value (μmax), and the descending gradient (Δμ) of traction coefficient (μ), vary according to the contact pressure between the rollers at a spot of contact between the roller outer peripheral surfaces. Specifically, as the contact pressure between the rollers increases, (1) ideal slip speed (Vo) decreases to broaden the range of negative gradient, (2) the maximum value (μmax) of traction coefficient (μ) becomes small, and (3) the descending gradient (Δμ) of traction coefficient (μ) becomes rapid in addition to the broadening of the range of negative gradient, thereby increasing the amount of descending of traction coefficient (μ) in the range of negative gradient.
In a conventional roller-type friction transmission unit, the feature that the roller outer peripheral surfaces are in contact with each other only at one point, and the inter-roller contact pressure at that point is high, results in that the μ-V characteristic has the tendencies (1)-(3) described above as indicated by a solid line in FIG. 5, so that smooth power transmission is difficult in the range of negative gradient of the μ-V characteristic, wherein judder (noise, vibration) tends to occur between the rollers.